The most appropriate one-word substitution for the given group of words "A fictitious name used by an author" is "pseudonym."

Let's provide a detailed explanation for each option:

1. Namesake: A namesake refers to a person who shares the same name as another person, typically in honor or as a tribute. It does not specifically relate to the practice of authors using fictitious names.

2. Homonym: A homonym refers to words that have the same pronunciation or spelling but different meanings. It does not directly relate to the practice of authors using fictitious names.

3. Pseudonym: A pseudonym is a fictitious name used by an author instead of their real name. Authors often use pseudonyms for various reasons, such as to protect their identity, adopt a different writing style, or experiment with different genres. The use of a pseudonym allows authors to separate their personal identity from their literary work.

4. Alibi: An alibi refers to evidence or proof that shows a person was not present at the scene of a crime or an event when it took place. It does not relate to the practice of authors using fictitious names.

Therefore, in the given context, the most appropriate one-word substitution for "A fictitious name used by an author" is "pseudonym," as it specifically refers to the practice of authors using false names for their literary works.